Side collision

How do I add side collision to this platformer?

init = function()
  ct.init()
  cx = 0
  cy = 0
  cz = 1
  map = ["map1"]
  level = 0
  velocy = 0
  velocx = 0  
end

update = function()
  if checkCollision(cx,cy,map[level],1536,1536) == false then
    velocy += .25
  end
  if checkCollision(cx,cy,map[level],1536,1536) == true then
    velocy = 0 
  end
  if keyboard.RIGHT and velocx < 3 then
    velocx += .25
  elsif keyboard.LEFT and velocx > -3then
    velocx -= .25
  elsif velocx < 0 then velocx += 1
  elsif velocx > 0 then velocx -= 1 end
    
  if keyboard.UP and checkCollision(cx,cy,map[level],1536,1536) == true or keyboard.SPACE and checkCollision(cx,cy,map[level],1536,1536) == true then 
    velocy = - 5
  end
  cx += velocx
  cy -= velocy
  ct.camera.glideTo(cx,cy,cz)
end

draw = function()
  screen.clear("rgb(0,170,255)")
  ct.drawMap(map[level],0,0,1536,1536)
  screen.drawSprite("hero",0,0,24,24)
end